Clément Chigot <[email protected]> writes: > On Fri, Oct 24, 2025 at 10:35 AM Markus Armbruster <[email protected]> wrote: >> >> Kevin Wolf <[email protected]> writes: >> >> > Am 03.09.2025 um 09:57 hat Clément Chigot geschrieben: >> >> This allows more flexibility to vvfat backend. The value for "Number of >> >> Heads" and "Sectors per track" are based on SD specifications Part 2. >> >> This is too terse to remind me of how vvfat picks cylinders, heads, and >> sectors before this patch, so I need to go dig through the source code. >> I figure it depends on configuration parameters @floppy and @fat-type >> like this: >> >> floppy fat-type cyls heads secs cyls*heads*secs*512 >> false 12 64 16 63 31.5 MiB >> false 16 1024 16 63 504 MiB >> false 32 1024 16 63 504 MiB >> true 12 80 2 18 1440 KiB >> true 16 80 2 36 2880 KiB >> true 32 80 2 36 2880 KiB >> >> How exactly does the new parameter @size change this? > > My prime goal was to create a 256 Mib VVFAT disk. As you can see, > today for hard-disks there are only two possibilities: 31.5 Mib or 504 > Mib. Hence, I've introduced the option `size=xxx` to allow more > granular choices. > This option changes how cyls, heads and secs parameters are computed > to be as closed as possible of its value. > > I did try to keep it simple. I could have introduced options to select > cylinders, heads, etc. But I think "size=xxx" would be more intuitive. > There are also approximations made, as not all sizes can be reached. I > didn't add errors or warnings for them. I'm fine adding them.
I don't have an opinion on whether we should support more sizes and/or provide full control over CHS geometry. >> >> Some limitations remains, the size parameter is recognized only when >> >> "format=vvfat" is passed. In particular, "format=raw,size=xxx" is >> >> keeping the previously hardcoded value: 504MB for FAT16 and 32 MB for >> >> FAT12. FAT32 has not been adjusted and thus still default to 504MB. >> >> 31.5MiB unless I'm mistaken. > > True, I will fix it. > >> I'm not sure what you're trying to convey in this paragraph. As far as >> I can tell, you're adding a @size parameter to vvfat, so of course it >> doesn't affect raw. > > Yes, but AFAICT, `if=sd,format=raw` will result in vvfat backend being > called. I didn't manage to make the new option work with > `if=sd,format=raw,size=256Mb`. Thus, when the "size" option is not > provided, I keep the previous value (those for your above comment). > Hence this paragraph to mostly warn people about the current > limitation. Are you talking about -drive? Complete examples, please. I'm confused about the connection between SD (from if=sd here, and "SD specification" above) and vvfat. SD is a frontend. vvfat is a backend. [...]
